Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lorenzo Stoakes]

Small thing, sorry to be a pain buuuut could we please not send patches
in-reply to another mail, it makes it harder for people to see :)

On Fri, Oct 31, 2025 at 11:59:16AM +0200, Mike Rapoport wrote:
> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > The error path in secretmem_fault() frees a folio before restoring its
> > direct map status, which is a race leading to a panic.
>
> Let's use the issue description from the report:
>
> When a page fault occurs in a secret memory file created with
> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
> the underlying page as not-present in the direct map, and add it to
> the file mapping.
>
> If two tasks cause a fault in the same page concurrently, both could
> end up allocating a folio and removing the page from the direct map,
> but only one would succeed in adding the folio to the file
> mapping. The task that failed undoes the effects of its attempt by (a)
> freeing the folio again and (b) putting the page back into the direct
> map. However, by doing these two operations in this order, the page
> becomes available to the allocator again before it is placed back in
> the direct mapping.
>
> If another task attempts to allocate the page between (a) and (b), and
> the kernel tries to access it via the direct map, it would result in a
> supervisor not-present page fault.
>
> > Fix the ordering to restore the map before the folio is freed.
>
> ... restore the direct map
>
> With these changes
>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

Agree with David, Mike this looks 'obviously correct' thanks for addressing
it.

But also as per Mike, please update message accordingly and send v2
not-in-reply-to-anything :P

With that said:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

>
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Reported-by: Google Big Sleep <big-sleep-vuln-reports@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@xxxxxxxxxxxxxx/
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >  mm/secretmem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > index c1bd9a4b663d..37f6d1097853 100644
> > --- a/mm/secretmem.c
> > +++ b/mm/secretmem.c
> > @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> >  		__folio_mark_uptodate(folio);
> >  		err = filemap_add_folio(mapping, folio, offset, gfp);
> >  		if (unlikely(err)) {
> > -			folio_put(folio);
> >  			/*
> >  			 * If a split of large page was required, it
> >  			 * already happened when we marked the page invalid
> >  			 * which guarantees that this call won't fail
> >  			 */
> >  			set_direct_map_default_noflush(folio_page(folio, 0));
> > +			folio_put(folio);
> >  			if (err == -EEXIST)
> >  				goto retry;
> >
> > --
> > 2.49.0
> >
>
> --
> Sincerely yours,
> Mike.